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Old 10-26-2009, 09:10 PM   #1
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Default FDR Gearing Question

How does the car react with the same FDR (or close to it) but with different spur/pinion combos.

Spur Pinion FDR (2:0 Ratio)
78 - 31 - 5.03
81 - 32 - 5.06
83 - 33 - 5.03
86 - 34 - 5.06


All the combos are very close in FDR -

What is the difference in performance and how the car accelerates?
Which will have hotter motor temps?
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Old 10-26-2009, 09:14 PM   #2
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There is no difference. The only thing that really changes, is a) how far forward or rearward the motor sits, and b) a tiny tiny tiny amount of weight and rotational mass if you use smaller gears.
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Old 10-26-2009, 09:24 PM   #3
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sorry brandon, have to disagree.

I believe there is difference in feel to the car/truck, a larger spur will have less wheel spin and better accelleration, while a smaller spur will hold corner speed better. drag brake effect falls in there somewhere also.

take katies truck sunday, brushed motor, red spur, (xxxt) motor ran hot, green (yellow) spur motor temp dropped same gear close as possible gear ratio.

I have asked the "pros" this very question, and depending on which one, some will say, no diff, some will say otherwise. its hard for me to put into words, but I will try harder and show you at the track
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Old 10-27-2009, 09:23 AM   #4
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sorry brandon, have to disagree.

I believe there is difference in feel to the car/truck, a larger spur will have less wheel spin and better accelleration, while a smaller spur will hold corner speed better. drag brake effect falls in there somewhere also.

take katies truck sunday, brushed motor, red spur, (xxxt) motor ran hot, green (yellow) spur motor temp dropped same gear close as possible gear ratio.

I have asked the "pros" this very question, and depending on which one, some will say, no diff, some will say otherwise. its hard for me to put into words, but I will try harder and show you at the track
Id have to say then, that there was something ELSE affecting Katies truck, one way or another then.... perhaps gear mesh, perhaps some other kind of issue with the gear itself, but i would bet GOOD money, it wasn't from changing the relative size of the gears.
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Old 10-27-2009, 10:32 AM   #5
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I would tend to disagree with you on this one, Cap'n. Inertia and kinetic momentum aside, the larger radius spur takes less force to spin (Torque=Forcexradius). That force is opposed by the spur (Newton's third law) with an equal but opposite force. This (opposite) force however has a better leverage on the motor due to the pinion's larger radius (to keep the FDR the same, the pinion has to increase as well). This means the spur becomes more efficient at slowing down the motor. Ergo, the motor has to develop higher forces at the periphery of larger pinions to achieve the same result, therefore the motor is worked harder.

The situation can be further complicated by settings in the ESC (drag brake, timing advance etc) which alter the way the motor behaves and what regime of strain it undergoes when the car slows down, accelerates or just coasts along.

Relating to the above there is the question of whether the spur radius increases slower with the difference in number of teeth than the pinion (i.e. the radius difference between a spur of 78 and one of 83 is larger than the difference in radius between correspponding pinions, i.e. 31-33). I tend to think the pinions get bigger quicker as the ratio of variation to number of teeth would suggest. That means that as the spur and pinion increase in size, the spur will have an increasingly higher leverage on the motor (whose leverage on the spur grows as well but slower). Briefly put, the motor is best at small pinion sizes. The larger the pinion, the less force the motor will impart at the pinion's periphery.

Practically, I am not sure this would have measurable consequences in the world of R/C when spur/pinion differences are as small as those above, but theory tends to suggest some loss of efficiency would happen.

To our friend who opened the topic I would suggest experimenting with ESC settings to see if motor temperatures can be kept down that way at any spur/pinion combo whilst maintaining the FDR.
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Old 10-27-2009, 10:36 AM   #6
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Old 10-27-2009, 10:57 AM   #7
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I would tend to disagree with you on this one, Cap'n.
Ok... im not sure I understand all that... but i can't imagine 1 or 2 teeth in either direction, in an RC application, could do something like dramatically increase or decrease motor temps.
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Old 10-27-2009, 11:04 AM   #8
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I never noticed any significant temp difference for such a small gear change, but what I did notice was corner speed. The bigger the spur, the more rotating mass, the more corner speed. This is overcome somewhat by the weight of the pinion. But we are dealing with a 4lb car, a few grams will have an effect. But the same is true going to a smaller spur. Quicker acceleration, even with the same gear ratio.

This is just my findings, other people may have found something different.
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Old 10-27-2009, 11:11 AM   #9
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I never noticed any significant temp difference for such a small gear change, but what I did notice was corner speed. The bigger the spur, the more rotating mass, the more corner speed. This is overcome somewhat by the weight of the pinion. But we are dealing with a 4lb car, a few grams will have an effect. But the same is true going to a smaller spur. Quicker acceleration, even with the same gear ratio.

This is just my findings, other people may have found something different.
Right... thats the only thing I could think of as well... rotational mass.
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Old 10-27-2009, 04:58 PM   #10
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Right... thats the only thing I could think of as well... rotational mass.
Even then, the difference in mass is TINY. I weighed a 72 and a 87 tooth Kimbrough spurs. The difference didn't even register on the scale. Granted this was a kitchen scale so not hyper accurate. I think the perceived difference is in between the ears of the beholder.
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Old 10-27-2009, 05:01 PM   #11
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Even then, the difference in mass is TINY. I weighed a 72 and a 87 tooth Kimbrough spurs. The difference didn't even register on the scale. Granted this was a kitchen scale so not hyper accurate. I think the perceived difference is in between the ears of the beholder.
Yep... which leads me right back to my first answer of.... if there IS even the most microscopic of differences, I can't imagine seeing or feeling that around an RC racetrack.
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Old 10-27-2009, 05:33 PM   #12
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A smaller spur and pinion will give you more torque but will also lose corner speed off throttle.
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Old 10-27-2009, 10:45 PM   #13
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question, who runs without a rear wing, why not, what difference could it make in such a small application such as a RC car? ummm

size is all relative, what possible difference could there be in 30 wt and 35 wt shock oil, what possible difference could there be in 14 xray spring and a hpi 14 lb spring, what possible difference could a bump steer washer make, point is it all makes a difference.

seeing how i set the mesh on katies truck I'm gonna say the mesh was fine.

talk to scotty or bridgewater, they can explain better.
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Old 10-28-2009, 12:26 AM   #14
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Just weighed
114 Spur was 5.0 grams
78 Spur 2.7 grams

Not much to consider alot of mass.
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